Zeolites are crystalline aluminosilicates which have a uniform crystal structure characterized by a large number of regular small cavities interconnected by a large number of even smaller rectangular channels. It was discovered that, by virtue of this structure consisting of a network of interconnected uniformly sized cavities and channels, crystalline zeolites are able to accept for absorption molecules having sizes below a certain well defined value whilst rejecting molecules of larger size, and for this reason they have come to be known as “molecular sieves.” This characteristic structure also gives them catalytic properties, especially for certain types of hydrocarbon conversions.
The ZSM family of zeolites is well known and their preparation and properties have been extensively described. Thus, for example, one type of the ZSM family or zeolites is that known as ZSM-5. The crystalline aluminosilicate zeolite known as ZSM-5 is particularly described in U.S. Pat. No. 3,702,886; the disclosure of which is incorporated herein by reference. ZSM-5 crystalline aluminosilicate is characterized by a silica-to-alumina mole ratio of greater than 5 and more precisely in the anhydrous state by the general formula:[0.9±0.2M2/nO:Al2O3:>5SiO2]wherein M having a valence n is selected from the group consisting of a mixture of alkali metal cations and organo ammonium cations, particularly a mixture of sodium and tetraalkyl ammonium cations, the alkyl groups of which preferably contain 2 to 5 carbon atoms. The term “anhydrous” as used in the above context means that molecular water is not included in the formula. In general, the mole ratio of SiO2 to Al2O3 for a ZSM-5 zeolite can vary widely. For example, ZSM-5 zeolites can be aluminum-free in which the ZSM-5 is formed from an alkali mixture of silica containing only impurities of aluminum. All zeolites characterized as ZSM-5, however, will have the characteristic X-ray diffraction pattern set forth in U.S. Pat. No. 3,702,886 regardless of the aluminum content of the zeolite.
Based on the unique pore structure of ZSM-5, this zeolite can be applied extensively as a catalyst material to various processes. Zeolite ZSM-5 has been shown to be a particularly useful catalyst in reactions involving aromatic compounds, with emphasis on those having a single carbocycle. Thus ZSM-5 exhibits unique selectivity in the conversion of olefins, naphthenes, alcohols, ethers and alkanes into aromatic compounds and in such reactions as isomerization, alkylation, dealkylation and transalkylation of aromatics. That favorable influence on aromatic conversion reactions is found also in the forms of ZSM-5 in which another metal appears in isomorphic substitution for aluminum, as described in U.S. Pat. No. 4,163,028. ZSM-5 has also been extensively applied in catalytic cracking and catalytic dewaxing. When ZSM-5 is used in catalytic cracking of petroleum, enhancement of gasoline octane is achieved. Accordingly, ZSM-5 has been used as an additive to other cracking catalysts, e.g. zeolite Y, to improve gasoline octane and LPG yields.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite within the range of 1:10 to 3:1.
The addition of a separate additive catalyst comprising one or more members of the ZSM-5 type has been found to be extremely efficient as an octane and LPG yield improver when used in very small amounts in conjunction with a conventional cracking catalyst. Thus, in U.S. Pat. No. 4,309,179, it was found that only 0.1 to 0.5 weight percent of a ZSM-5 type catalyst added to a conventional cracking catalyst under conventional cracking operations could increase octane by about 1 to 3 RON+0 (research octane number without lead).
Generally, the octane gain of a ZSM-5 containing cracking catalyst is associated with gasoline (C5+) yield decrease and correspondingly higher yields of C3 and C4 gaseous products. As the freshly added ZSM-5 undergoes hydrothermal deactivation, the octane enhancement is reduced and additional ZSM-5 must be added to maintain the desired octane level.
Crystalline aluminosilicates in general have been prepared from mixtures of oxides including sodium oxide, alumina, silica and water. More recently clays and coprecipitated aluminosilicate gels, in the dehydrated form, have been used as sources of alumina and silica in reaction systems. In some instances of synthetic faujasite synthesis from clay, the zeolitic product is in the form of an aggregate.
U.S. Pat. No. 4,091,007 teaches a method of preparing ZSM-5 from preformed extrudates without losing the shape of the extrudates upon crystallization. The extrudate contains a mixture of silica sources such as Ludox and sodium silicate, and kaolin calcined at 1800° F. In cases where it contains raw kaolin, the extrudate has been calcined in the temperature range between 1700° F. and 2000° F. Organic templates, such as tetramethylammonium chloride, tetrapropylammonium bromide, tri-n-propylamine, and n-propyl bromide, were used during crystallization in the examples. Extradates containing up to 60% of grown ZSM-5 were produced. A similar process is described in U.S. Pat. No. 5,558,851.
EP Publication No. 0,068,817 reveals a method of making ZSM-5 from acid-leached metakaolin. Metakaolin is treated with a strong acid, e.g., hydrochloric acid, sulphuric acid, and nitric acid, and orthophosphoric acid to extract at least part of the aluminum oxide content of the metakaolin and provide the material with a SiO2/Al2O3 (mol) ratio in the range of from 10-200:1. In the presence of a quaternary compound such as tetrapropylammonium hydroxide, the acid-treated metakaolin reacts with NaOH to provide ZSM-5.
U.S. Pat. No. 6,004,527 teaches the synthesis of ZSM-5 from preformed silica-only microspheres. Aluminum and sodium sources were added via incipient wetness impregnation using the respective nitrate salts. Tetrapropylammonium hydroxide was used as a directing agent. ZSM-5 crystallinity of the product was 25% and the particle shape and size of the silica microsphere were retained in the product.
U.S. Pat. No. 4,522,705 relates to a method of making ZSM-5 additive catalyst prepared by in-situ crystallization of a clay aggregate. Clay microspheres were treated with an aqueous solution of sodium hydroxide and organic template such as n-propylamine. In a variation, the preformed microspheres were formed containing crystalline ZSM-5 as seeds. Crystallization was carried out in the presence of NaOH and n-propylamine.
Seeding as a means for inducing crystallization is a very old technique. In the art of zeolite manufacture, various patents describe the use of seeding to induce the rapid crystallization of zeolites. Various patents describing the manufacture of zeolite crystals by seeding with a zeolite include: United Kingdom Pat. No. 1,297,256, in making ZSM-4; U.S. Pat. No. 3,247,194, in making ZK-5; U.S. Pat. No. 3,733,391, in making faujasite; and U.S. Pat. No. 4,007,253, in making faujasite in which the seed is not the same as the product. Patents disclosing the formation of zeolites by seeding with other aluminosilicates include: United Kingdom Patent No. 1,117,568, in making ZSM-4; United Kingdom Patent No. 1,160,463, in making faujasite; U.S. Pat. No. 3,578,398, in making a zeolite similar to offretite; and U.S. Pat. No. 3,947,482, in making various zeolites. It is to be understood that the mentioned preceding patents are not an exhaustive list of all patents which discuss forming zeolite crystals by seeding.
EP Publication No. 0,156,595 teaches in-situ ZSM-5 synthesis from extrudates and microspheres in the absence of any organic compounds of nitrogen and phosphorus. 5% ZSM-5 is included as seeds in the preformed particles. The seeds employed are said to be the same as the zeolite intended to grow. In examples where there is no ZSM-5 seed presence, no ZSM-5 is crystallized.
U.S. Pat. No. 5,145,659 discloses the synthesis of ZSM-5 from a preformed matrix such as clay extrudates or spray dried microspheres containing a source of silica, alumina, alkali metal, or mixtures thereof. The preformed matrix is enriched in silica by precipitation of silicate from a solution of a silicate source and addition of a zeolite synthesis reaction mixture which contains an organic templating agent and/or ZSM-5 seeds. No ZSM-5 is crystallized in examples where there are no ZSM-5 seeding crystals or organic template present.
U.S. Pat. No. 6,261,534 discloses a zeolite crystallization method that comprises combining a template-free reaction mixture containing a source of silicon oxide and aluminum oxide and sufficient water to shape the mixture into particles. Zeolites such as ZSM-5 are crystallized within the shaped particles while heating the reaction mixture in the absence of an external liquid phase. While there is disclosure that ZSM-5 can be crystallized without the addition of seeds, ZSM-5 crystals are used in the only examples disclosed.
U.S. Pat. No. 5,232,675 discloses the synthesis of a rare earth-containing high-silica zeolite having pentasil-type structure. The reaction mixture is free of organic compounds of nitrogen and phosphorus. Water glass, aluminum phosphate, and inorganic acid are used as raw materials, and REY or REX zeolite are used as seeds. Reaction data suggests that upon steam deactivation, the rare-earth containing ZSM-5 have good activity maintenance as compared to ZSM-5 without rare earth.
Chinese patent publications CN 1,194,943A and CN 1,057,067C disclose a process for synthesizing molecular sieve ZSM-5 with use of NaY mother liquid as raw material and includes the acid deposition of silica and alumina, spray drying to obtain microspheres, mixing the microspheres with NaOH, water and optionally crystal seed of molecular sieve, and hydrothermal crystallizing.
Since the 1960's, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, called microspheres, containing both an active zeolite component and a non-zeolite component in the form of a high alumina, silica-alumina matrix.
In prior art fluid catalytic cracking catalysts, the active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in-situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components.
For many years a significant proportion of commercial FCC catalysts used throughout the world have been made by in-situ synthesis from precursor microspheres containing kaolin that had been calcined at different severities prior to formation into microspheres by spray drying.
For example, commonly assigned U.S. Pat. No. 4,493,902, the teachings of which are incorporated herein by cross-reference, discloses novel fluid cracking catalysts comprising attrition-resistant, high zeolitic content, catalytically active microspheres containing more than about 40%, preferably 50-70% by weight Y faujasite and methods for making such catalysts by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. In a preferred embodiment, the microspheres containing the two forms of calcined kaolin are immersed in an alkaline sodium silicate solution, which is heated, preferably until the maximum obtainable amount of Y faujasite is crystallized in the microspheres.
In carrying out the invention described in the '902 patent, the microspheres composed of kaolin calcined to undergo the exotherm and metakaolin are reacted with a caustic enriched sodium silicate solution in the presence of a crystallization initiator (seeds) to convert silica and alumina in the microspheres into synthetic sodium faujasite (zeolite Y). The microspheres are separated from the sodium silicate mother liquor, ion-exchanged with rare earth, ammonium ions or both to form rare earth or various known stabilized forms of catalysts. The technology of the '902 patent provides means for achieving a desirable and unique combination of high zeolite content associated with high activity, good selectivity and thermal stability, as well as attrition-resistance.
While the prior art as discussed above has attempted to form in-situ ZSM-5 from clay aggregates, including aggregates formed from kaolin, the zeolite-forming process has included the addition of organic templates typically used in ZSM-5 synthesis and/or required use of ZSM-5 seed crystals. The need to use either of these materials greatly undercuts the economic advantage of using kaolin aggregates as a raw material in zeolite synthesis by the in-situ method.